Summary: Recorded workers' remittances to developing countries have grown rapidly, to more than $100 billion in 2004, bringing increasing attention to these flows as a potential tool for development. But even these statistics are likely to significantly understate true remittances, as a large share is believed to flow through informal channels. Estimates of the importance of the informal sector vary widely, ranging from 35 percent to 250 percent of total remittances. The primary motivation of the authors is to develop the first empirical methodology to estimate informal flows. They use insights from the literature on shadow economies and empirically estimate informal remittances for more than 100 countries using historical data on the balance of payments (BOP), migration, transaction costs, and country characteristics. Their results imply that informal remittances amount to about 35-75 percent of official remittances to developing countries. There is significant regional variation: informal remittances to Sub-Saharan Africa and Eastern Europe and Central Asia are relatively high, while those to East Asia and the Pacific are relatively low. These estimates are supplemented with detailed household survey data on remittance receipts in a number of countries. The results also shed light on the determinants of recorded remittances and the associated fees in the formal sector. The authors find that the stock of migrants in OECD countries is the primary determinant of remittances. In addition, money transfer fees and the presence of dual exchange rates reduce the share of remittances reported in national accounts. In turn, transaction costs are systematically related to concentration in the banking sector, lack of financial depth, and exchange rate volatility. There is also evidence that remittances are misrecorded in the BOP as "errors and omissions."